It will be appreciated by those skilled in the art that rheostats and potentiometers are closely related with a rheostat or variable resistance equivalent to a potentiometer where the output terminal and one of the two reference terminals are employed. In both cases there is a variable resistor.
In the case of a digitally programmable resistor, the value of resistance is set in response to a digital input value.
A common type of digital rheostat employs a resistor string. The programmable nature of the resistance is obtained by having each node in the string coupled by a switch to an output terminal.
With a single resistor string, as the word length or resolution of the digital input increases, the required number of switches and resistors also increases with the number of switches and resistors equating to 2N where N is the length in bits of the digital word. The word length is also referred to commonly as the binary resolution of the resistor string.
Correspondingly, the incremental difference in resistance decreases as the word length increases for a given total resistance. It will be appreciated that this equates to the resistance for the LSB.
Multi-stage digitally programmable resistors are known in which the resistance value is the series combination of a plurality of different resistance stages with a first stage converting the Least Significant Bits (LSBs) into a resistance value and the second stage converting the Most Significant Bits (MSBs) into a resistance value optionally including one or more intermediate stages handling the intermediate bits of the digital input word. This approach reduces the number of resistors and switches required. This provides for improved circuit performance while reducing integrated circuit area and reducing test cost.
However, the resistance corresponding to that of the LSB remains the same. As the length of the digital input word increases, for a given target total resistance value, the resistance of the LSB decreases with the result that the ON resistance of switches Ron becomes significant and distorts the overall resistance value.
Accordingly, if a circuit designer is looking to employ either one or both of a low target total resistance or higher resolution, the switch resistances can significantly impact on performance.
This is particularly so in thR case of multi-stage programmable resistors, where there are multiple switches coupled in series between the two terminals which presents limitations on the performance of multi-stage architectures.